25-Aug-14 Simple Java I/O Part I General Principles

2 Prologue “They say you can hold seven plus or minus two pieces of information in your mind. I can’t remember how to open files in Java. I’ve written chapters on it. I’ve done it a bunch of times, but it’s too many steps. And when I actually analyze it, I realize these are just silly design decisions that they made. Even if they insisted on using the Decorator pattern in java.io, they should have had a convenience constructor for opening files simply. Because we open files all the time, but nobody can remember how. It is too much information to hold in your mind.” —Bruce Eckel,

3 Streams All modern I/O is stream-based A stream is a connection to a source of data or to a destination for data (sometimes both) An input stream may be associated with the keyboard An input stream or an output stream may be associated with a file Different streams have different characteristics: A file has a definite length, and therefore an end Keyboard input has no specific end

4 How to do I/O import java.io.*; Open the stream Use the stream (read, write, or both) Close the stream

5 Why Java I/O is hard Java I/O is very powerful, with an overwhelming number of options Any given kind of I/O is not particularly difficult The trick is to find your way through the maze of possibilities open use close

6 Opening a stream There is data external to your program that you want to get, or you want to put data somewhere outside your program When you open a stream, you are making a connection to that external place Once the connection is made, you forget about the external place and just use the stream open use close

7 Example of opening a stream A FileReader is a used to connect to a file that will be used for input: FileReader fileReader = new FileReader(fileName); The fileName specifies where the (external) file is to be found You never use fileName again; instead, you use fileReader open use close

8 Using a stream Some streams can be used only for input, others only for output, still others for both Using a stream means doing input from it or output to it But it’s not usually that simple--you need to manipulate the data in some way as it comes in or goes out open use close

9 Example of using a stream int ch; ch = fileReader.read( ); The fileReader.read() method reads one character and returns it as an integer, or -1 if there are no more characters to read The meaning of the integer depends on the file encoding (ASCII, Unicode, other) open use close

10 Manipulating the input data Reading characters as integers isn’t usually what you want to do A BufferedReader will convert integers to characters; it can also read whole lines The constructor for BufferedReader takes a FileReader parameter: BufferedReader bufferedReader = new BufferedReader(fileReader); open use close

11 Reading lines String s; s = bufferedReader.readLine( ); A BufferedReader will return null if there is nothing more to read open use close

12 Closing A stream is an expensive resource There is a limit on the number of streams that you can have open at one time You should not have more than one stream open on the same file You must close a stream before you can open it again Always close your streams! open use close